The present invention relates to a method and apparatus for forming an image by belt transfer, which are used in an electrophotographic copying machine, printer, and the like, and transfer medium chucking method.
In particular, the present invention relates to a method and apparatus for forming an image by belt transfer, which are used in a four-drum tandem color copying machine, printer, and the like, and a transfer medium chucking method of electrostatically chucking a sheet with a transfer belt by charging the surface of the transfer belt by applying charges to it.
As a transfer technique in an electrophotographic image forming apparatus, a method using a corona charger opposing a photosensitive body is known.
This method, however, generates toxic ozone. As an ozone-free transfer technique, one employing a contact method has been proposed.
Japanese Patent Laid-Open No. 6-110343 discloses a technique in which a semiconductive transfer belt and a transfer roller disposed behind the transfer belt are used and transfer is performed by applying a transfer bias to the transfer roller.
As a color image forming apparatus for forming a color image by using a plurality of toners, i.e., Y (yellow), M (magenta), C (cyan), and Bk (black), the following methods are known:
(1) A method of forming four color images on one photosensitive body to superpose them on each other and transferring them at once.
(2) A transfer drum method of holding a transfer medium on a transfer drum and forming a four-color image by rotating the transfer drum four times.
(3) An intermediate transfer body method of forming four color images on an intermediate transfer body and transferring them on a transfer medium at once.
(4) A four-drum method with which four photosensitive bodies are arranged parallel to each other and a four-color image is formed while a transfer medium passes among them.
According to a color image forming apparatus employing the four-drum method, color images formed on four parallel image carriers can be transferred onto a transfer medium in a multiple manner while the transfer medium passes among them, thereby forming a color image. Therefore, an image can be formed with a time xc2xc that required by other processes that form a four-color image. This apparatus is suitable for a higher speed process.
In a color image forming apparatus employing the four-drum method that forms Y, M, C, and Bk toner images with four parallel photosensitive drums, a process unit for forming a monochromatic image is provided, so the service life of the process units for forming color images can be prolonged, leading to a cost reduction.
When forming a monochromatic image, a transfer belt and first, second, and third color image forming photosensitive bodies are separated from each other, and only the fourth black image forming photosensitive body is brought into contact with the transfer belt. Then, the operations of the process units for forming color images are stopped. The transfer belt and color image forming photosensitive bodies are separated from each other in order to prevent wear of the transfer belt and color image forming photosensitive bodies.
In the four-drum type image forming apparatus, since four photosensitive bodies are arranged parallel to each other, the distance for which the transfer medium is conveyed by the transfer belt is, e.g., 300 mm or more. When forming a monochromatic image, if the transfer belt and the color image forming photosensitive bodies are separated from each other, after the transfer medium is fed, it must be conveyed on the transfer belt.
A sheet chucking unit is required to stably convey the transfer medium on the transfer belt. In a conventional chucking unit, a transfer belt and transfer medium are sandwiched together, and charges are applied by a charger, thereby chucking the transfer medium.
With this chucking unit, the charges applied to the surface of the transfer belt for the purpose of sheet chucking are discharged in the transfer section to the upper surface side of the photosensitive body. As a result, charges of opposite polarity to that of the image forming charges applied to the photosensitive body are applied. This causes defective charging in the photosensitive body to lead to an image defect.
It is an object of the present invention to provide a method and apparatus for forming an image, which are capable of preventing an image defect caused by defective charging which occurs when charges applied to the surface of a convey belt for the purpose of sheet chucking are discharged to the surface of a photosensitive body, and a transfer medium chucking method.
An apparatus for forming an image according to the present invention comprises a plurality of image carriers on which a toner image is to be formed, a developing unit for forming a toner image on the image carriers, a convey belt disposed in contact with the image carriers and adapted to convey a transfer medium, a chucking unit for electrostatically chucking the transfer medium onto the convey belt, and a transfer unit for transferring the toner image from the image carriers to the transfer medium in a transfer region where the image carriers are in contact with the convey belt, the chucking unit having a charger for applying charges to a surface of the convey belt, and a countercharge applying unit for applying countercharges necessary to electrostatically chuck the transfer medium onto the convey belt, wherein the charger starts charge application to the surface of the convey belt such that that portion on the surface of the convey belt to which the charges are applied by the charges reaches the countercharge applying unit at substantially the same timing as a timing at which a leading end of the fed transfer medium reaches the countercharge applying unit, and the charger ends charge application to the surface of the convey belt such that that portion on the surface of the convey belt to which the charges have are applied passes through the countercharge applying unit at substantially the same timing as a timing at which a trailing end of the transfer medium passes through the countercharge applying unit.
The apparatus may have a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, in the monochromatic mode, the convey belt and, of the plurality of image carriers, an image carrier on which a toner image is to be formed with a color toner may be separated from each other, and the transfer medium may be chucked onto the convey belt by the chucking unit only in the monochromatic mode.
An apparatus for forming an image according to the present invention comprises a plurality of image carriers on which a toner image is to be formed, a developing unit for forming a toner image on the image carriers, a convey belt disposed in contact with the image carriers and adapted to convey a transfer medium, a chucking unit for electrostatically chucking the transfer medium onto the convey belt, and a transfer unit to which a bias is applied in a transfer region where the image carriers are in contact with the convey belt, to transfer the toner image from the image carriers to the transfer medium, the transfer unit being electrically grounded while the bias is not applied thereto.
The apparatus may have a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, in the monochromatic mode, the convey belt and, of the plurality of image carriers, an image carrier on which a toner image is to be formed with a color toner may be separated from each other, and the transfer unit may be electrically grounded only in the monochromatic mode while the bias is not applied thereto.
An image forming apparatus according to the present invention may comprise a plurality of image carriers on which a toner image is to be formed, a developing unit for forming a toner image on the image carriers, a convey belt disposed in contact with the image carriers and adapted to convey a transfer medium, a chucking unit for electrostatically chucking the transfer medium onto the convey belt, and a transfer unit to which a bias is applied in a transfer region where the image carriers are in contact with the convey belt, to transfer the toner image from the image carriers to the transfer medium, wherein the apparatus may have a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, and in the monochromatic mode, the convey belt and, of the plurality of image carriers, an image carrier on which a toner image is to be formed with a color toner may be separated from each other, and the chucking unit may chuck the transfer medium onto the convey belt, and a first bias for transferring the toner image may be applied to the transfer unit while the transfer image is being transferred from the image carrier to the transfer medium, and a second bias lower than the first bias may be applied to the transfer unit while transfer of the toner image is not performed.
Alternatively, an image forming apparatus according to the present invention comprises a plurality of image carriers on which a toner image is to be formed, a first charging unit for applying charges to surfaces of the image carriers, a developing unit for forming a toner image on the image carriers, a convey belt disposed in contact with the image carriers and adapted to convey a transfer medium, a chucking unit which has a second charging unit for applying charges to a surface of the convey belt and a countercharge applying unit for applying countercharges in order to electrostatically chuck a transfer medium with the convey belt, and electrostatically chucks the transfer medium onto the convey belt, and a transfer unit for transferring the toner image from the image carriers to the transfer medium in a transfer region where the image carriers are in contact with the convey belt, wherein the charges applied to the surfaces of the image carriers by the first charging unit and the charges applied to the surface of the convey belt by the second charging unit have the same polarity.
An image forming apparatus according to the present invention also comprises a plurality of image carriers on which a toner image is to be formed, a developing unit for forming a toner image on the image carriers, a convey belt disposed in contact with the image carriers and adapted to convey a transfer medium, a chucking unit which has a charging unit for applying charges to a surface of the convey belt and a countercharge applying unit for applying countercharges in order to electrostatically chuck a transfer medium with the convey belt, and electrostatically chucks the transfer medium onto the convey belt, and a transfer unit to which a bias is applied in a transfer region where the image carriers are in contact with the convey belt, to transfer the toner image from the image carriers to the transfer medium, wherein the apparatus has a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, in the monochromatic mode, the convey belt and, of the plurality of image carriers, an image carrier on which a toner image is to be formed with a color toner are separated from each other, and a DC bias is applied to the charging unit while the fed sheet passes near the countercharge applying unit from a leading end to a trailing end thereof, in order to apply charges to the surface of the convey belt, and an AC bias is applied to the charging unit in a remaining period.
According to the present invention, there is provided a transfer medium chucking method of applying charges to a surface of a convey belt by using a charger, and applying countercharges to the convey belt by using a countercharge applying unit, thereby electrostatically chucking a transfer medium onto the convey belt, the method comprising starting charge application to the surface of the convey belt with the charger such that that portion on the surface of the convey belt to which the charges are applied by the charges reaches the countercharge applying unit at substantially the same timing as a timing at which a leading end of the fed transfer medium reaches the countercharge applying unit, and ending charge application to the surface of the convey belt with the charger such that that portion on the surface of the convey belt to which the charges are applied passes through the countercharge applying unit at substantially the same timing as a timing at which a trailing end of the transfer medium passes through the countercharge applying unit.
The method may have a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, and the transfer medium may be chucked onto the convey belt by the charger and the countercharge applying unit only in the monochromatic mode.
According to the present invention, there is provided an image forming method of forming a toner image on a surface of an image carrier, conveying a transfer medium with a convey belt disposed in contact with the image carrier, and transferring the toner image with a transfer unit to which a bias is applied, from the image carrier to the transfer medium in a transfer region where the image carrier is in contact with the convey, wherein the transfer unit is electrically grounded while the bias is not applied thereto.
The image forming method may have a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, and the transfer unit may be electrically grounded while the bias is not applied thereto only in the monochromatic mode.
According to the present invention, there is provided an image forming method of forming a toner image on a surface of an image carrier, conveying a transfer medium with a convey belt disposed in contact with the image carrier, and transferring the toner image with a transfer unit to which a bias is applied, from the image carrier to the transfer medium in a transfer region where the image carrier is in contact with the convey belt, wherein the method has a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, and in the monochromatic mode, a first bias for transferring the toner image is applied to the transfer unit while the transfer image is being transferred from the image carrier to the transfer medium, and a second bias lower than the first bias is applied to the transfer unit while transfer of the toner image is not performed.
According to the present invention, there is provided a transfer medium chucking method of applying charges to a surface of an image carrier, where a toner image is to be formed, by using a first charging unit, applying charges to a surface of a convey belt which conveys a transfer medium, by using a second charging unit, and applying countercharges to the convey belt by using a countercharge applying unit, thereby electrostatically chucking the transfer medium onto the convey belt, wherein the charges applied to the surfaces of the image carrier by the first charging unit and the charges applied to the surface of the convey belt by the second charging unit have the same polarity.
Alternatively, according to the present invention, there is provided a transfer medium chucking method of applying charges to a surface of a convey belt by using a charger, and applying countercharges to the convey belt by using a countercharge applying unit, thereby electrostatically chucking a transfer medium onto the convey belt, wherein the method has a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, and in the monochromatic mode, a DC bias is applied to the charging unit while the fed sheet passes near the countercharge applying unit from a leading end to a trailing end thereof, in order to apply charges to the surface of the convey belt, and an AC bias is applied to the charging unit in a remaining period.
In this manner, the present invention has one of the following arrangements:
(1) The convey belt is charged only while a sheet is placed on it and chucked onto it.
(2) The transfer means for applying a transfer bias is grounded while a transfer bias is not applied, and an electric field is formed on the transfer means side, so that discharge from the surface of the convey belt to the photosensitive body side is suppressed.
(3) While transfer is not performed, a bias lower than the bias applied during transfer is applied to the transfer means, so the potential of the transfer means becomes lower than the surface potential of the convey belt.
(4) The polarity of the charges used to charge the convey belt is set to be equal to that of charges used to charge the photosensitive body.
(5) While chucking a sheet, the surface of the convey belt is charged. During the remaining period, an AC bias is applied to the convey belt in order to remove charges from the surface of the convey belt.
Normally, the charges applied to the surface of the transfer belt for the purpose of chucking the sheet are discharged to the photosensitive body surface side to cause defective charging, leading to an image defect. According to the present invention, the problem of image defect can be solved with one of the above arrangements.